gfauto_cov: fix gcc 9+ support (#953)
gfauto_cov: fix gcc 9+ support (#953)

Support for gcc 9+ was broken, as it had never been tested. The JSON output from gcov is all on one line, so processing line-by-line is not possible. Instead, we now just parse the JSON using `json.loads(...)`.
google/graphicsfuzz,google/graphicsfuzz,google/graphicsfuzz,google/graphicsfuzz,google/graphicsfuzz,google/graphicsfuzz,google/graphicsfuzz